The invention disclosed herein relates generally to mechanical rotary drive mechanisms, and more particularly to drive trains of the type which are driven in at least one direction by an electric motor and driven in the opposite direction by a spring return device.
In ventilating and air conditioning systems it is common to utilize dampers and/or valves for controlling the temperature, humidity and distribution of a fluid medium used in controlling conditions in an air conditioned space. Where air is used as the fluid medium, dampers may be used for mixing varying proportions of air at different temperatures and/or humidities within the air conditioning system, mixing air within the system with variable amounts of outside air for achieving operating economies, and distributing conditioned air to areas within the conditioned space.
The dampers or valves in air conditioning systems and other devices in a variety of applications may be positioned by means of an electric motor which is operable in one or both directions. In systems of the type which employ a single direction motor the motor and an associated drive train are used to move the damper or other device in one direction, and a return spring is typically used to produce movement in the opposite direction. In systems of the type which employ a reversible motor a return spring may be employed to provide return operation in the event of electrical power failure. The characteristics of the motor and the gearing ratio of the drive train may be such that the spring is unable to return the damper or other device as long as the motor is mechanically engaged with the drive train. Thus, when it is desired to move the damper or other device by means of the return spring, the drive train must be disengaged from the motor.
With the motor decoupled from the drive train, and absent some provision for controlling the speed of the drive train, the drive train may reach an excessive return speed before being halted abruptly by a limiting position stop. Particularly if the drive train and/or devices connected thereto have substantial inertia, components may be broken, distorted or weakened by the abrupt stop. Therefore, it is common to provide some means for governing the return speed. Many actuators of the type described employ oil filled gear boxes. In such designs, the viscosity of the oil fill may provide adequate speed control. Further, the speed governing action of the oil fill may be enhanced by paddle wheel arrangements in the drive train.
It is also known to provide mechanical speed governors in which an assembly rotated by the drive train includes movably mounted weights which are displaced outwardly by centrifugal force when the rotational speed of the assembly exceeds a predetermined value. Movement of the weights may be used to bring a portion of the rotating assembly into contact with a stationary surface to achieve a speed retarding effect. In simplest form, the weights themselves may be configured with surfaces which frictionally engage a stationary surface.
The primary driving, decoupling and speed governing functions of a drive mechanism can be performed by separate elements. However, for purposes of minimizing complexity of the mechanism and reducing its weight, size and cost, it is desirable to provide an implementation in which multiple functions are performed by a reduced number of components. Accordingly, the applicant has produced a unique drive mechanism design in which primary drive, decoupling and speed governing functions are achieved with a minimum number of simple multiple function components.